Scientists believe the UK ranges of two aggressive alien crustaceans - the North American signal crayfish and the Chinese mitten crab - are beginning to overlap, offering the prospect of an epic battle for supremacy over Britain's waterways.
The North American signal crayfish (Pacifastacus leniusculus) is one of several species introduced into the UK for aquaculture in the 1970s, and which have since made themselves right at home.
The signal crayfish poses a particular threat to "native aquatic plants, invertebrates and fish through direct predation", but also carries the fungal disease crayfish plague (Aphanomyces astaci), which could ultimately wipe out our own white-clawed crayfish (Austropotamobius pallipes).
Ecologist Stephanie Peay, describing the immigrant crustaceans as "aquatic thugs", told the BBC: "The plague does not seem to harm the non-natives, but if our native white-clawed crayfish encounter it, they die within weeks."
She added: "Where the non-natives move in, the white claws are lost. Survey work has shown that it only takes between four and seven years from first arrival to achieve a complete local extinction. The only future for the white claws is in isolated water bodies that are completely free from non-native crayfish."
However, the signal crayfish's bid for freshwater domination could soon be challenged by the Chinese mitten crab (Eriocheir sinensis) - which put its first tentative claw into European waters back in the 1930s, but has launched a full-scale invasion since the 1980s.
The BBC explains that the Chinese mitten crab probably hitches a lift to the UK in ships' ballast water. The crabs' offspring are born in river estuaries, but then migrate upstream before returning years later to saltwater to breed. During their travels, they can wander up to 1,500km (930 miles), according to Chinese studies. In the process, they lay waste to pretty well everything in their path, as Paul Clark from the Natural History Museum explained: "They eat anything that they can get their claws into: weeds, fish eggs, snails, molluscs - anything. They are just an opportunistic feeder."
Clark admitted it was difficult to estimate the exact number of Chinese mitten crabs currently ravaging Britain's freshwater courses, but suspected it was "in the millions".
Now, scientists are nervously awaiting the apparently inevitable clash of crustaceans when the two species finally meet. Current locations for a possible first encounter are southern England's River Lee and Yorkshire's River Ouse and River Aire.
Aquatic ecologist Philine zu Ermgassen, of the University of Cambridge, told the Beeb: "Mitten crabs are spreading so quickly at the moment. They are now moving further up rivers into areas where there are crayfish - it seems they are starting to encroach upon crayfish terrain."
Ms zu Ermgassen appeared to favour an ultimate cancrine victory, should the two aliens cross claws. She explained: "You could predict a clash between the two. If this is the case, the mitten crabs do seem to be more dominant as a species - the crabs are very aggressive and very strong. They will also be directly competing for food."
In the end, even if the signal crayfish and Chinese mitten crab decide not to rumble in the aquatic arena of death, their combined presence will prejudice local ecosystems. Stephanie Peay concluded: "What tends to happen when successful invasive non-native species move in is that you tend to get a reduction in the diversity of native species and a reduction in abundance. That will be bad for the native environment overall." ®
What the ecologists haven't factored in here is the possibility that rampaging black widow hordes will take exception to their fellow invaders, leading to a devastating arachno-crustacean armageddon as immigrant armies battle for the title of "most obnoxious non-native species".